Various devices exist and have been proposed in the art for use in capturing energy from marine environments and converting this to electricity. For example, turbine systems have been proposed which convert the energy from tidal flows, wave motion, marine currents and the like. The methods and apparatus used to appropriately deploy and support such devices within a marine environment are of critical importance, particularly as most deployment sites are characterized by extreme environmental conditions, and thus high energy.
In some known arrangements energy conversion devices are supported via rigid structures, such as columns or frames, which are mounted on the seabed. Although such arrangements may provide robust anchoring they typically require complex installation, such as preparing suitable foundations, which might be difficult to achieve, especially in extreme environmental conditions. Further, these seabed mounted rigid structures may be difficult to deploy in larger water depths.
It is also known in the art to utilise mooring techniques for station keeping of subsea devices, such as tidal turbines. In one known arrangement a mooring line is secured between a seabed anchor and a conventional buoy which by virtue of its buoyancy applies tension to the mooring line, wherein a turbine is tethered to the tensioned line to be maintained in a suitable operational position. However, drag and other forces applied through the tethered turbine on the mooring line are typically of such magnitude that extremely large buoy volumes are necessary to apply appropriate tension within the mooring line. If appropriate buoyancy is not provided then this may result in large deviations of the subsea device and the mooring line, for example relative to the seabed anchor, which is undesirable and may result in the device not being held on station. Large buoy volumes, however, create additional drag and introduce unnecessary stresses and strains on the mooring line. All of this results in unfavourable dynamic motions being induced on the mooring line, which are ultimately transmitted to the tidal turbine and impact negatively on device stability and operational performance. Furthermore, the volume and hence associated buoyancy of such a buoy has to be sized for the maximum tidal velocity experienced at a site of deployment, meaning the buoy will be oversized for the vast amount of operational time. This over-sizing approach introduces additional parasitic drag which has to be compensated for by increasing the size and thus the holding power of the mooring anchors and line. This may increase the complexity, size, weight and costs of the system, and may create additional handling issues such as during transportation, deployment and maintenance.